1. Field of the Disclosure
The present disclosure relates to optical disk drives and systems having optical disk drives, and, more particularly, to the control of optical disk drives.
2. Description of the Related Art
Optical disk drives are very popular compact storage devices due to their low cost and the high reliability of the optical disk storage. Optical disks have high reliability because there is no wear associated with their repeated use, thus providing write once, read many (WORM) capability. Optical disk drives use transmitted electromagnetic waves, generally in the visible light spectrum, for recording and retrieval of information. Optical disk drives record and detect specific variations in the optical properties of the media surface. The most common optical disk formats include CD, CDLaser ROM, CD-R, CD-RW, DVD-ROM, DVD+RW, DVDRW, DVD-RAM, and 5¼″ Magneto Optic.
Data is stored on tracks on the optical disks. Actuators in the optical disk drives must acquire and maintain focus, seek to a given track and then maintain the relative position in a track while data is written or read. Sensors measure the position of an optical pickup unit. The measured position is used to correct any positioning errors.
FIG. 1, labeled prior art, illustrates a portion 100 of an optical disk drive. A sled 102 holds an optical pickup unit (OPU) 104 and moves along sled tracks 106 controlled by a sled motor 108. Sled motor 108 provides coarse movement of sled 102. OPU 104 holds magnets 110 and 112, current carrying wires 114, and coils 116 and 118 for providing fine control of the movement of a lens assembly 105 in OPU 104. By providing current on wires 114, which are coupled to coils 116 and 118, magnetic forces are created that move lens assembly 105 in nearly orthogonal directions. Magnet 110 and coil 116 control the movement of lens assembly 105 in an X direction, herein referred to as a tracking direction. Magnet 112 and coil 118 control the movement of lens assembly 105 in a Z direction, herein referred to as a focus direction. Note that sled 102 and OPU 104 include other devices not shown for simplicity of illustration.
The main control loops of optical disk drive 100 include a coarse (lower frequency) tracking loop to position sled 102 in the vicinity of the desired tracks on the optical media, a focus loop to control the distance between lens assembly 105 and the media, and a fine (higher frequency) tracking loop to lock lens assembly 105 onto the track position. Optical disk drive 100 includes multiple other control loops, for example, a servo loop to control the speed of the spindle.
FIG. 2, labeled prior art, illustrates a system model 200 of one of multiple servo systems of an optical disk drive. Control laws 202 are used to produce, for example, focus commands. Control laws 202 can be analog or digital, include gain factors, digital or analog filtering functions, non-linear and logic operators and the like. In the case of digital control, these control signals can be converted, for example, using a digital to analog converter (DAC), to produce appropriate signals to control, via focus actuators 206, the position of lens assembly 105. Disturbances and noise 208 affect the resulting or actual position of lens assembly 105. A sensor 210 senses the actual position of lens assembly 105 as a measured position. However, disturbances and noise 212 affect the measurement of the position by sensor 110 and result in a measured position that is different than the actual position. The measured position is subtracted from a reference or desired position 214 and input to control laws 202. Control laws 202 are designed to minimize this difference between the measured position and the desired position. Note that due to disturbances and noise 208 and 212, what is commanded is not what actually occurs; and what actually occurs is not what is measured.
System model 200 has been described according to a focus servo system of an optical disk drive. Other servo systems, for example, a tracking servo system, a sled servo system, and the like can have similar system models.
Disturbances and noise 208, 212 come from a variety of sources. Disturbances and noise 208 can be significant and are primarily due to mechanical runnout of the disk, disk warping, imperfections in the physical spacing of the tracks, cross-coupling of the tracking and focus actuators, and external movement or jarring of the optical drive. Disturbances and noise 212 can be due to sensor cross-coupling, A/D converter noise, amplifier (thermal noise), and the like. Disturbances are predominately periodic or predictable in nature with an uncorrelated (noise) component, whereas noise is typically a random process.
Typically, in the prior art, these disturbances can be handled by increasing the control system bandwidth by using higher order digital filter and faster sampling rates. However, this increases the cost of the system, for example, due to the cost of higher performance processors required to execute a larger number of operations at a faster rate. Increasing the control system bandwidth also increases the overall system's sensitivity to small variations (lot-to-lot manufacturing tolerance variations) in the electro-mechanical characteristics of the optical pickup units. This increased sensitivity to manufacturing variations either increases the cost of the optical pickup units to maintain a constant manufacturing yield (via the use of higher quality optical pickup units with lower lot-to-lot variations) or decreases the manufacturing yield of the completed optical disk drive with the original, higher variance optical pickup units.
Better performing optical disk drive control systems are desired that do not increase the cost of the system or reduce manufacturing yields.
The use of the same reference symbols in different drawings indicates similar or identical items.